Conventional electric power supply devices and systems, such as batteries, fuel cells, solar cells, are used to provide stable and continuous electric power for a variety of electrical devices. These batteries or fuel cells usually involve converting chemical energy or light energy into electrical energy, and are indispensable in many portable electronic circuit applications. These types of power supplies however suffer from short storage life spans due to high electric leakage. Thus they do not have a long standby time without being recharged or replaced. In addition, the physical sizes of these types of batteries are difficult to be miniaturized, and the costs are relatively high due to their delicate internal structures.
For electric energy storage, capacitors are also not suitable to store energy for a long time due to electric leakage. As such, conventional batteries, fuel cells and capacitors are not preferred for some applications by which long energy standby time is needed, such as portable Radio Frequency IDentification (RFID) devices, particularly those used for tamper indicating electric seals. The attachment of a conventional battery particularly does not make sense for RFID tags used for tamper indicating electric seals, in which low but long standby power is utilized only temporally for transmitting the RF signals, sometimes once in life. No currently available power supply device or system is suitable for such temporal, low power supply but with long standby time. The continuing success in RFID systems and applications is being limited by the lack of efficient and appropriate power supply device.
Many energy harvesting mechanisms have been explored for RF systems and applications, including devices and systems capable of converting optical, magnetic, thermal, and mechanical energy into electricity. However, these types of energy conversion devices and systems have relatively complicated structures. In addition, these types of devices and systems are not suitable for use in applications where no other type of energy source is available. For example, a mechanical source is not available for a stationary object, an optical source is not available for items stored in the dark, etc. This is particularly the case in which, RF tags are to be used for tamper indicating electric seals.